A random-access list implementation based on Chris Okasaki's approach
on his book "Purely Functional Data Structures", Cambridge University
Press, 1998, chapter 9.3.

RAList is a replacement for ordinary finite lists.
RAList provides the same complexity as ordinary for most the list operations.
Some operations take O(log n) for RAList where the list operation is O(n),
notably indexing, '(!!)'.